What is the difference between BI systems and forecasting systems?

Depending on the size and scope of your company, I agree with you: there are somefunctions a BI product (such as Business Objects) can perform for the budgeting and forecasting cycle. However, things like cash flow forecasting, smoothing, distributed risk analysis, and other non-scalar functionality quickly erodes in a true reporting tool. A product like Adaptive Planning, or others of similar ilk, will be a good addition to your business as you grow.

Web analytics are a different game altogether. I don't think you'll find products that take that into account without joining (not knowing the size of your business) a CoreMetrics or Omniture web management tool, or even Google analytics, with a traditional process. I suggest looking at conversion rates, ad CPM, and clickthrough rates as ratios and doing a consensus-based budget with aggressive, conservative, and likely scenarios. Then use thee ratios as the base data for decision system in a true B&P tool. Feel free to message me if you'd like to dig deeper into this, and good luck!